The present invention relates to an apparatus for reading optically recorded data, and more particularly, to an apparatus for reading optically recorded data having a servo system adapted to maintain the optical axis of a recorded data reading light beam perpendicular to the recording surface of a recording medium.
If the angle between the optical axis of a recorded data reading light beam and a recording medium (recording disk) deviates significantly from 90.degree., "leakage" of data from adjacent recording tracks occurs that is, cross talk occurs.
A variety of factors can cause the abovedescribed angular deviation. For instance, such deviation may be caused if the recording disk becomes warped or if the disk rotating shaft is bent. These types of difficulties occur after shipment of the product, and therefore the occurrence thereof is unavoidable.
Accordingly, it is necessary to provide a servo system which electrically detects the presence of cross talk and in response maintains the optical axis of the recorded data reading light beam perpendicular to the recording disk, thereby to minimize the amount of cross talk. Such a technique is disclosed by Japanese Laid-Open Patent Application No. 186237/1982. The technique disclosed by this Japanese Patent Application is applied to a reproducing device for a recording disk on which data is recorded according to a CLV (constant linear velocity) system. More specifically, cross talk is detected utilizing the fact that, in a recording track produced using the CAV (constant angular velocity) system, the synchronizing signal recording intervals are aligned on the same radial line throughout the recording tracks. This is not true, however, for a recording track produced using the CLV system. For the CAV system, the leakage of synchronizing signal data from adjacent tracks is detected thereby to detect the amount of cross talk, and the optical axis of the data reading light beam is controlled so as to eliminate the cross talk thus detected.
The above-described technique is disadvantageous in that the electrical circuit needed for detecting the cross talk is complex and expensive, and the technique is applicable only to recording disks produced using the CLV system, not those produced using the CAV system.
In accordance with another technique, as disclosed by Japanese Laid-Open Patent Application No. 179954/1982 and as outlined in FIG. 1 herein, an objective lens 3 is provided to focus a recorded data reading light beam with an optical axis 1 on the recording surface 2 of a recording disk. A light source 4 for emitting a detecting auxiliary beam is provided to permit detection of the amount of deviation of the angle between the optical axis 1 and the recording surface from 90.degree.. The auxiliary beam is applied through the objective lens 3 to the recording surface 2, and the light reflected therefrom is applied through the objective lens 3 to a pair of light-detecting elements 5 and 6. When the recording surface 2 is perpendicular to the optical axis 1, the auxiliary beam is reflected symmetrically with respect to the optical axis and is thus returned symmetrically with respect to the optical axis, as indicated by a solid line in FIG. 1. On the other hand, if the recording surface is inclined, as indicated by the dot/dash line or the broken line, extending through the recording surface 2, then the auxiliary beam will be reflected as indicated by the dot/dash line or the broken line extending through the objective lens 3.
The light-detecting elements 5 and 6 are provided for these reflected beams, as shown in FIG. 1. The outputs of the light-detecting elements 5 and 6 are applied to respective inputs of a differential amplifier 7, which provides a detection output. The detection output is supplied to a subtractor 8 where the detection output and a reference signal 9 are subjected to subtraction to provide a subtraction output. The subtraction output is applied to an optical axis inclination control mechanism 10 to cause the optical axis to form a 90.degree. angle with respect to the recording surface.
In the above-described technique, the auxiliary beam, which is a parallel beam, is applied through the objective lens to the recording surface. Due to this fact, the technique suffers from the following difficulties: Unless the focus servo system operates very stably, the servo system used for making the optical axis 1 of the recording surface (hereinafter referred to as "a tilt servo system" when applicable) cannot operate properly. Therefore, the tilt servo operations cannot be effected before the focus servo is locked in. At the worst, neither the tilt servo operation nor the focus servo can be effected. In other words, in order for the auxiliary beam emitted parallel to the optical axis 1 to be reflected to the correct positions on the light-detecting elements 5 and 6, the disk surface must coincide with the focal point of the lens. If the disk surface is not at the focal point of the lens, the positions on the light-detecting elements to which the reflected auxiliary beam is applied are unsuitable, being shifted to the right or left in FIG. 1. Therefore, the differential amplifier 7 provides an erroneous output so that the angle of the optical axis 1 is set incorrectly.
If the angle of the optical axis deviates too much from 90.degree., the position of a data detection beam reflected to a detector (not shown) adapted to supply a control signal to the focus servo system or the tracking servo system is changed, and therefore unwanted DC components are superposed on an error signal used for locking in the servo system.
Moreover, since the pickup is miniaturized, it is difficult to arrange the optical system of the tilt servo system in the pickup. That is, since the auxiliary beam is applied to the objective lens provided for the recorded data reading light beam, the angle of field of the lens for the recorded data reading light beam is small, which limits the design of the optical system.
In order to eliminate the above-described difficulties, a device as shown in FIGS. 2 and 3 has been proposed by the present applicant and disclosed in Japanese Utility Model Application No. 63397/1983. In FIG. 2, a disk 11 is inclined with respect to the horizontal line due, for instance, to warping of the disk. An optical head unit 12 for optically reading data on the recording disk is provided. The optical head unit 12 is rockably mounted on a shaft 14 supported by supporting members 13. The supporting members 13 are fixedly secured to a slider 15 which is adapted to move the head unit 12 radially of the disk 11. A part of the slider 15 is formed into a rack 16 engaged with a pinion gear 17. The pinion gear 17 is rotated by a slider motor 18 so that the slider 15 is moved radially of the disk.
Furthermore, an ordinary control-signal generating section 20 and a high-speed control-signal generating section 21 are provided. The ordinary control signal generating section 20 detects the DC component included in an error signal outputted by a tracking error signal generator (not shown) to provide a signal for controlling the movement of the slider. The high-seed control-signal generating section 21 produces a high-speed control signal during recorded data address retrieval, that is, during scanning operations. The outputs of these two signal generating sections are applied to an adder 22, and the output of the adder 22 is applied to a driver circuit 19 to operate the slider motor 18.
In order to detect the inclination of the disk 11, a light-emitting element 23 and light-detecting elements 24a and 24b are provided on the head unit 12. The outputs of the light-detecting elements 24a and 24b are applied to a differential amplifier 25, the difference output of which is applied to a driver circuit 26 to operate a tilt motor 27. A male-threaded part 28 coupled to the rotary shaft of the tilt motor 27 is engaged with a female-threaded part provided in a part of the optical head unit 12 in such a manner that the latter is inclined with rotation of the tilt motor 27. In this operation, the optical head unit 12 is rocked about the abovedescribed shaft 14 supported by the supporting members 13. A spring S placed around the male-threaded part 18 is provided to prevent backlash.
FIG. 3 is a perspective view of the optical head unit 12. In FIG. 3, reference numeral 29 designates an objective lens. A recorded data reading light beam emitted from a laser source built into the unit 12 is focused on the recording surface of the recording disk 11 by the objective lens 29. The center of the objective lens 29 is set at the intersection of the optical axis 31 of the light beam and the shaft 14. A focus actuator 30 includes a magnetic circuit, coils, etc.
A straight line that connects the center of the objective lens 29 and the center of the light-emitting element 23 is substantially parallel to the tangent of a recording track which is being reproduced. The light-emitting element 23 is preferably set ahead of the data detecting point where the light beam is focused in the recorded data reading operation so that light emitted from the light-emitting element and its reflected light will not pass through the objective lens 29.
The optical axis 31 of the recorded data reading light beam is parallel to the optical axis of the light beam from the light-emitting element 23. Even if the former optical axis is in alignment with the latter optical axis, no problem will be caused.
The operation of the device thus designed will be described with reference to FIGS. 4A through 6C. FIGS. 4A through 4C show reflection states of a light beam from the light-emitting element 3 for different inclination angles of the disk, FIGS. 5A through 5C show the different reflection states as viewed from the side of the disk, and FIGS. 6A through 6C indicate the outputs a and b of the light-detecting elements 24a and 24b, and the output c of the differential amplifier 25. More specifically, FIGS. 6A through 6C indicate the output a of the light-detecting element 24a, the output b of the light-detecting element 24b, and the output c of the differential amplifier 25. The points A, B and C in FIGS. 6A through 6C correspond to the states shown in FIGS. 4A and 5A, 4B and 5B, and 4C and 5C, respectively. The point B corresponds to the case where the disk is oriented horizontally, perpendicular to the optical axis 31.
In the case where the disk is perpendicular to the optical axis 31, the light beam from the light-emitting element 23 is reflected by the recording surface of the disk 11 while diverging, and the light beam thus reflected is applied uniformly to the light-detecting elements 24a and 24b. Therefore, the outputs of the two elements 24a and 24b are equal in magnitude, and hence the output level of the differential amplifier 25 is zero.
On the other hand, if the disk 11 is warped as shown in FIG. 6A or 6B, the disk 11 is no longer perpendicular to the optical axis 31. In the case of FIG. 4A, the reflected light is applied only to the light-detecting element 24a, and therefore the output level of the light of the light-detecting element 24b is substantially zero, as a result of which the output level of the differential amplifier 25 is maximum with a positive polarity. Similarly, in the case of FIG. 4C, the reflected light is applied only to the light-detecting element 24b, and therefore the output level of the differential amplifier 25 is maximum with a negative polarity.
Accordingly, the output c of the differential amplifier 25 is a signal having a level and polarity which change with the deviation of the angle between the disk 11 and the optical axis and the direction of the deviation. Therefore, if a closed-loop tilt servo operation is carried out so that the differential output c is zeroed, the optical axis 31 of the light beam can be maintained perpendicular to the disk 11 at all times and thus cross talk can be eliminated. If the light from the light-emitting element 23 has an intensity distribution such as a Gaussian distribution, the slope of the output characteristic of the differential amplifier is large and the detection sensitivity is high, as a result of which the size and arrangement of the light-detecting elements 24a and 24b can be determined readily.
The differential output of the differential amplifier 25 drives the tilt motor 27 in such a manner that the optical head unit 12 is rocked about the shaft 14. Tilt servo operation is carried out in this manner.
However, in the above-described device, the diffraction of the light beam reflected from the recording disk is not taken into considertation. In actuality, it has been found that the diffracted light is affected in configuration, position and intensity by the pit configuration and the track pitch of the disk. As a result, data other than the tilt data tends to be mixed into the tilt signal, that is the tilt signal may become erroneous.
FIG. 7 is a plan view showing the light source 23 and the light-detecting elements 24a and 24b arranged without taking the effect of the primary diffracted light into consideration. In FIG. 7, reference numerals 32 and 33 designate the primary diffracted light beams. In this case, the recording track extends in the direction indicated by the dot/dash line in FIG. 7. In an actual recording disk, the pits are arranged two-dimensionally, and therefore the light is diffracted in the vertical direction in FIG. 7. However, in that direction, the pits are arranged irregularly due to the signal information they convey, and the effects thereof can be substantially disregarded. On the other hand, the track pitch is relatively high in regularity, and therefore, with respect to the light beams diffracted perpendicularly to the track, the effect of the track pitch cannot be disregarded. The track, being spiral, has a certain curvature. Therefore, in practice, the right and left first-order diffracted light beams are asymmetrical to a degree determined by this curvature (in FIG. 7, the diffracted light beams are shown as being symmetrically for convenience in illustration), and their configurations are affected by the distance between the disk and the light-detecting elements 24a and 24b.
When the above described positive and negative first-order diffracted light beams 32 and 33 are applied to the light-detecting elements 24a and 24b, due to differences in disk conditions such as different track curvatures of the inner and outer peripheries of the disk and variations in intensity of the positive and negative first-order diffracted light beams due to the asymmetric configuration of the pits, data other than the data of inclination of the disk, namely, noise signals, are mixed in so that the tilt signal becomes erroneous, thus making it difficult to perform the tilt servo operation with a high accuracy.